Means for measuring speed or distance

ABSTRACT

A device for generating and utilizing voltage pulses for determining the revolutions per minute of a rotating shaft or the number of complete cycles per minute performed by a reciprocating or oscillating shaft, having a solid state radiation source, a semi-conductor element sensitive to the radiation and an opaque member having an aperture therein to permit radiation to reach the element for generating a voltage pulse, the opaque element being driven in synchromism with the rotating, reciprocating or oscillating shaft, whereby by fast switching and electronic summation of the voltage pulses the revolutions or cycles can be counted.

United States Patent 1191 Ford et al.

[ MEANS FOR MEASURING SPEED OR DISTANCE [75] inventors: Eric HaroldFord, London; Brian Christopher Tate, Downham, Bromley, both of England[73] Assignee: Lumenition Limited, London,

England [22] Filed: Jan. 5, 1971 [211 App]. No.: 104,070

Related U.S. Application Data [63] Continuation-impart of Ser. No.864,230, Feb. 26,

i969, abandoned.

[30] Foreign Application Priority Data Feb. 29, 1968 Great Britain9,945/68 Feb. 28, 1969 France 6905420 Feb. 27, 1969 Germany..... P 19 098863 Feb. 26, 1969 Italy 858457 A/69 Feb. 28, i969 Japan 45/15258 Feb.26, I969 Sweden.... 2644/69 Feb. 28, l969 Canada.... .044239 Feb. 28,1969 Austria 51256 [52] U.S. Cl. 324/175, 250/231 SE, 250/233 [51] Int.Cl. G0lp 3/48 [58] Field of Search 324/175, 16], 172,

324/162; 250/233;-307/3l 1; 73/185, 187, l94 E, 229, 230, 231; 330/19;250/23l SE July 17, 1973 [56] References Cited UNITED STATES PATENTS3,421,103 1/1969 Nolde 330/19 3,156,115 11/1964 Adelmann..... 3,559,0651/1971 Grundy 324/175 OTHER PUBLICATIONS ity Component Resolver, J. Sci.Instr. 1965-42, pp. 414-417.

Primary Examiner-Michael J. Lynch Attorney-Larson, Taylor & Hinds [57]ABSTRACT A device for generating and utilizing voltage pulses fordetermining the revolutions per minute of a rotating shaft or the numberof complete cycles per minute performed by a reciprocating oroscillating shaft, having a solid state radiation source, asemi-conductor element sensitive to the radiation and an opaque memberhaving an aperture therein to permit radiation to reach the element forgenerating a voltage pulse, the opaque element being driven insynchromism with the rotating, reciprocating or oscillating shaft,whereby by fast switching and electronic summation of the voltage pulsesthe revolutions or cycles can be counted.

' 14 Claims, 6 Drawing Figures 0 3 5645 1* i IL 3' ll 1r l -2r & n

z; z; I: a; a:

SHEET 2 IF 3 1 MEANS FOR MEASURING SPEED OR DISTANCE CROSS REFERENCEcillating shaft and the utilization of the pulses so generated to effecta very accurate measurement.

DESCRIPTION OF THE PRIOR ART US. Pat. No. 2,084,267 discloses anignition system using the rotation of theengine to drive a shutter pasta light source so as to interrupt a light beam reaching a photo-cell.The photo-cell controls an input to an amplifier in accordance with thelight pulses reaching it. The amplifier pulse output is fed to the sparkplugs via a distributor.

SUMMARY OF THE INVENTION Since the device for generating the voltagepulses in order to determine the revolutions, oscillations orreciprocations of the shaft concerned has to be mounted near mechanicalmachinery, it is very important that the correct functioning of thedevice is not affected by vibrations. This is particularly the case ifthe device is used to measure the speed or engine R.P.M. of anautomobile.

It is therefore an object of the present invention to improve on theprior art constructions by including solid state elements for theradiation source and detector.

It is a further object of the invention to provide means for theutilization of the generated voltage pulses whereby the pulses are madeto fast switch in an inverse manner a cascade arrangement of solid stateelements and are then summated to give a reading on a meter which isdirectly proportional to the speed or engine R.P.M. of the automobile.

It is yet a further object of the invention to provide an alternativetype of opaque element to interrupt the electro-magnetic radiation.

According to the present invention there is provided a device forgenerating voltage pulses for determining the revolutions per minute ofa rotating shaft or the number of complete cycles per minute performedby a reciprocating or oscillating shaft, including a semiconductorelement sensitive to electromagnetic radiation which will switch on orconduct when exposed to radiation and switch off when the radiation iscut off; a solid state radiation source; an opaque element positionedbetween the radiation source and the semiconductor element; said opaqueelement having a single aperture; means for moving the opaque element intimed relation to the rotating, reciprocating or oscillating shaft; atransistorized amplifier connected to the output of the semiconductorelement, said amplifier having a plurality of stages, the semiconductorand each stage of the amplifier being arranged to switch inversely incascade whereby a voltage pulse is produced every time theelectromagnetic radiation is either presented to or cut off from thesemiconductor element regardless of the speed of rotation, reciprocationor oscillation of the shaft.

Preferably the solid state radiation source is a galliumarsenide lamp.The semiconductor element may be either a photo-transistor or a photodarlington pair.

The amplifier means preferably has two or three stages. The solid stateradiation source and the semiconductor element may both be provided witha stabilized voltage across them. Preferably, the stabilized voltage isprovided by a zener diode connected in parallel therewith.

In addition means may be provided for summating the output from theamplifier in order to obtain a direct reading of speed or distance.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will now bedescribed in greater detail by way of example with reference to theaccompanying drawings, wherein:

FIG. 1 shows a device for generating a voltage pulse for the measurementof the speed of rotation of a shaft;

FIG. 2 is a circuit diagram of one preferred form of transistorizedamplifier utilizing the output from the photo-transistor shown in FIG.1;

FIG. 3 is a circuit diagram of an amplifier and summator circuit forproviding a direct measurement of speed or distance from the output ofthe phototransistor shown in FIG. 1;

FIGS. 4 and 5 are diagrammatic views of alternative types of opaqueelement with a cut out portion to transmit the electro-magneticradiation; and

FIG. 6 is a circuit diagram of a modified form of transistorizedamplifier of simpler design to that shown in FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1 of thedrawings, the device comprises a stator 2 surrounding a rotor 4. Therotor 4 is connected to the shaft whose revolutions per minute are to bemeasured through the shaft 6. Centrally located in the top of the stator2 there is provided a bulb holder 8 containing a filament lamp 10 orgallium arsenide infra-red lamp. The lamp 10 projects inside acylindrieal cup 12 which is bored axially into the top of the rotor 4.The lamp 10 is derated by being connected in series with a resistor 14.The annular'wall of the rotor 4 in the region of the cylindrical cup 12has a small circular hole 16 bored in it.

In a chamber 17 in the wall of the stator 2 there is arranged aphoto-transistor or light sensitive semiconductor device 18. The chamber17 in which the phototransistor 18 is housed has an aperture 20 which isat the same height as the hole 16 in the rotor 4.

In operation, each time the rotor 4 rotates the hole 16 comes intoalignment with the aperture 20 for a briefinstant. During this instant,light from the lamp 10 is received by the photo-transistor 18 whichthereby causes its energization and the generation of a voltage pulse,which is thereafter amplifier (if necessary) before being applied to abinary or decimal counter stage in order to determine the number ofrevolutions the shaft has performed or alternatively to a suitableelectronic tachometer to indicate R.P.M. The device may also be used inconjunction with speedometers, odometers, anemometers and sea speedindicators.

Referring now to FIG. 2 of the drawings, the gallium arsenide infra-redlamp 10 and the photo-transistor 18 are supplied in parallel from al2-volt battery (not shown) through a resistor 22. The resistor 14 isconnected in series with the gallium arsenide source 10 so that the lampis run at a derated value. A resistor 24 is connected in series with theemitter-collector path of the photo-transistor 18. A zener diode 26 isconnected in parallel across the parallel circuit consisting of firstlythe lamp l and resistor 14 and secondly the phototransistor 18 andresistor 24. The zener diode 26 ensures that both these elements receivea stabilized voltage supply. The output from the photo-transistor 18 isapplied to the base electrode of a transistor 28 forming part of a threestage amplifier which also includes transistors 30 and 32 as the secondand third stages. The base electrodes of the transistors 30 and 32 areconnected to the 12 volt supply through resistors 34 and 36respectively. The photo-transistor 18 and the three transistors 28, 30and 32 of the three stage amplifier are arranged to switch inversely incascade as indicated by the table given below:

Electro-magnetic radiation 18 28 30 32 Not interrupted ON OFF ON OFFInterrupted OFF ON OFF ON The above described circuit is capable of veryfast switching and is capable of producing an output of 1 amp at afrequency of up to 100 KHZ. This output can be used to measure speed anddistance of a road vehicle.

The circuit of FIG. 3 discloses an amplifier having only two stages anda summator capable of integrating the output from the amplifier in orderto obtain with the aid of a micro-ammeter a direct measurement of speedor distance. In this arrangement the phototransistor 18 is replaced by aphoto darlington pair comprising a photo-transistor 38 and a transistor40, the photo-transistor 38 having a resistor 42 connected in itsemitter circuit. A pair of diodes 44 and 46 are connected in seriesbetween the commoned collector electrodes of the photo darlington pairand the base electrode of the transistor 28 constituting the first stageof the amplifier. These diodes 44 and 46 are provided to raise thevoltage across the darlington pair 38-40. Resistors 48, 50, 51 and 52are additionally included in the amplifier circuit which excludes thethird stage shown in the previous embodiment.

The summator circuit includes a transistor 54, a meter 58, a capacitor60, a diode 62, a zener diode 64 and resistors 56 and 66. The amplifyingtransistor 54 operates on the same inverse principle with respect to thetransistor adjacent to it as is the case with all adjacent transistorsin FIG. 2. The transistor 54 turns on when transistor 30 turns off andvice versa.

This operation can be briefly summarized by the following table:

Electra-magnetic radiation 38/40 28 3O 54 Not interrupted -ON OFF ON ONInterrupted OFF ON OFF OFF When the transistor 54 is turned on thecircuit com- I prising the capacitor 60, the diode 62 and the collec-When the transistor 54 is turned off the capacitor 60 is charged by theinrush current through the meter 58 and the resistor 66. The capacitorhas a small capacitance and due to the presence of the zener diode 64,every time the transistor 54 is switched off, the capacitor is rapidlycharged to the zener voltage regardless of the frequency of switching.Accordingly the quantity of current passed through the meter 58 isdirectly proportional to the inrush current through the meter when thetransistor 54 is turned OFF, i.e., during a fraction of the time thatthe hole 16 in the cup 12 allows infra-red radiation from the lamp 10 tofall on the photo-transistor 18. Thus the faster the repetition of thesquare wave input, the greater the current flow through the meter perunit time since the meter sums all the individual inrush spikes everytime the transistor 54 switches OFF for a brief interval. Therefore themeter reading is directly proportional to revolutions per unit time of arotating shaft, or the number of complete cycles per unit time performedby a reciprocating or oscillating shaft.

Referring to FIG. 4, instead of using the walls of the rotor cup 4 asthe opaque element the photo-transistor l8 and the infrared lamp 10 maybe placed on opposite sides of an opaque disc provided with a V- shapedslot 82 therein. The sides of the slot 82 are radially aligned withrespect to the disc.

In the alternative construction of FIG. 5 the mark space ratio of theswitching of the transistor 56 is made equal to unity by increasing theangular width of the slot 82 to 180.

Referring now to FIG. 6, the modified form of amplifier includes twotransistors 84 and 88. The transistor 84 has its collector electrodeconnected to the +12-volt supply through a resistor 86 and the resistor22. A shunt diode is provided across the emitter-collector circuit ofthe photo-transistor 18 to ensure clean switching and to prevent reversetransients from causing multiple switching or oscillation. In operationa 2 mA current through the photo-transistor 18 is switched and amplifiedto produce 20 mA and mA through the transistors 84 and 88 respectively.The circuit is otherwise identical in operation to that described inFIG. 2.

The above described device is thus not only very reliable but has a widefield of application. It may be used to determine:

a. Engine R.P.M. of Shaft R.P.M.

b. Road speed of a vehicle.

0. Distance run (marine or land vehicles).

(1. Total number of revolutions, reciprocations or oscillations of anyobject.

What we claim and desire to secure by Letters Patent is:

1. An apparatus for determining the frequency of occurrence ofa givenstateof a moving member comprising a semiconductor device sensitive toinfra-red radiation for generating a square wave voltage plusc byswitching on" when exposed to infra-red radiation and switching off"when the infra-red radiation is cut off; a solid state infra-redradiation source; an element opaque to infra-red radiation positionedbetween said source and said semiconductor device and having a singleaperture therein; means for moving the opaque element in timed relationto the moving member; a transistorized amplifier having a plurality oftransistor stages arranged to switch inversely in cascade between an onstate and an off state so that a transistor of one stage is alwaysconducting; means for connecting said amplifier to the semiconductordevice such that the stages of the amplifier switch inversely to producea square wave voltage pulse in synchronism with the square wave voltagepulse generated by the semiconductor device; electrical storage meanscharged from a source responsive to the switching off of the last stageof said amplifier; and means for measuring the average value of theinrush charging currents to said electrical storage means so as toproduce a direct reading of the frequency of occurrence of the state ofsaid moving member.

2. An apparatus according to claim 1, wherein the semiconductor devicecomprises a photo-transistor the base electrode of which is leftunconnected, and the solid stage infra-red source comprises a galliumarsenide lamp.

3. An apparatus according to claim 2, further comprising a zener diodeconnected in parallel with the gallium arsenide lamp and thephoto-transistor for stabilizing the voltage across said lamp and saidphototransistor, and a diode connected across the emittercollectorelectrodes of the photo-transistor for ensuring clean switching thereofand for preventing reverse transients from causing multiple switching ofthe phototransistor.

4. An apparatus according to claim 1, wherein said semiconductor devicecomprises a photo-transistor and transistor arranged as a darlingtonpair with commoned collector electrodes and the emitter electrode of thephoto-transistor connected to the base electrode of the transistor, andwherein a pair of series diodes is connected between the commonedcollector electrodes and thebase electrode of the first transistor ofthe amplifier.

5. An apparatus according to claim 1, wherein the opaque element is adisc having a cut out portion which extends through 180, whereby thesquare wave voltage generated by the semiconductor device has amarkspace ratio which equals unity.

6. An apparatus according to claim 1, wherein the last stage of saidamplifier comprises a transistor which switches on during the off"period in which the infra-red radiation is cut off from thesemiconductor device and which switches off during the on period inwhich infra red radiation reaches the semiconductor device, the meansfor integrating the output of the amplifier includes an ammeter, acapacitor and a diode, the diode and ammeter being connected in parallelwith one another, and the capacitor being connected between the outputof the last stage of the amplifier and the parallel combination of diodeand ammeter so that the emitter-collector circuit of the transistorforming the last stage of the amplifier short circuits the capacitor andmeter when said last stage transistor switches on, said capacitor beingcharged by the current which passes through the ammeter when thetransistor forming the last stage of the amplifier switches off.

7. An apparatus according to claim 2, wherein said opaque elementcomprises a rotor having a cylindrical cup at one end with a singleaperture therein, said gallium arsenide lamp extending into said cup andbeing fixed to a stator and said photo-transistor being housed in thewall of the stator in line with the rotating aperture in the cylindricalcup and said gallium arsenide lamp.

8. In combination, a road vehicle and an apparatus for determiningtheengine revolutions of the road vehi- 6 cle including a semiconductordevice sensitive to infrared radiation for generating a square wavevoltage pulse by switching on when exposed to the infra-red radiationand switching off when the infra-red radiation is cut off therefrom; asolid state infra-red radiation source; an element opaque to infra-redradiation having a single aperture formed therein, said opaque elementbeing positioned between said source and said semiconductor device;means for moving the opaque element in timed relation to the enginerevolutions of said road vehicle; a transistorized amplifier having aplurality of inversely switching transistor stages, means for connectingsaid transistorized amplifier to the semiconductor device such that thesemiconductor device and the transistor stages of the amplifier switchin versely in cascade between an on state and an of state so that atransistor of one stage is always conducting so as to produce a squarewave voltage pulse every time a pulse occurs from the output of thesemiconductor device; and electrical storage means charged from a sourceresponsive to the switching off of the last stage of said amplifier; andmeans for measuring the average value of inrush charging currents tosaid electrical storage means so as-to produce a direct reading of theengine revolutions of the vehicle.

9. The combination according to claim 8, wherein the means for effectinga summation of the pulses from the output of the amplifier includes anammeter, a capacitor, a diode, a zener diode and a transistor, means forconnecting said capacitor in series with said amplifier across thecollector-emitter path of the transistor, means for connecting saiddiode across said ammeter and zener diode across the collector-emitterpath of the transistor so that the transistor causes short circuiting ofthe capacitor when said transistor is turned on during the off period inwhich the infra-red radiation is cut off from the semiconductor device,said capacitor being charged by the current which passes through themeter when the transistor switches of during the onperiod in which theinfra-red radiation reaches the semiconductor device.

10. The combination according to claim 8, wherein said opaque membercomprises a rotor having a cylindrical cup at one end with an aperturein the wall thereof, said solid state source being fixed to a stator andbeing housed within said cup, and the semiconductor device being housedin the wall of the stator in line with the solid state source and in thesame plane as the aperture in the wall of the cup.

11. The combination according to claim 8, wherein the semiconductordevice comprises a photodarlington pair and the infra-red radiationsource comprises a gallium arsenide lamp, said combination furthercomprising a zener diode connected in parallel with a first series armcomprising the gallium arsenide lamp and a resistor, and a second seriesarm comprising a photo-darlington pair and a resistor, the three armedparallel circuit so formed being connected in series with a resistoracross a DC voltage source.

12. An apparatus for determining the speed of a road vehicle including aphoto-transistor sensitive to electromagnetic radiation which conductswhen exposed to radiation and switches off when the radiation is cutoff; a solid state radiation source; a disc, having a cut away, portionpositioned between the source and the phototransistor; means forrotating the disc in relation to engine speed so that radiation fromsaid source passing through the cut away portion of said disc to saidphototransistor causes a corresponding series of square wave voltagepulses to be produced at the output of the photo-transistor; atransistorized amplifier having a plurality of transistor stages, thestages of said amplifier being connected so as to switch inversely incascade in such a manner that at any one instant one transistor of theamplifier is always conducting; means for connecting the amplifier tothe photo-transistor such that the output produced by the amplifier is asquare wave which is in synchronism with the square wave voltage pulsesproduced at the output of the photo-transistor; and electrical storagemeans charged from a source recircumference of the disc.

1. An apparatus for determining the frequency of occurrence of a givenstate of a moving member comprising a semiconductor device sensitive toinfra-red radiation for generating a square wave voltage pluse byswitching ''''on'''' when exposed to infra-red radiation and switching''''off'''' when the infra-red radiation is cut off; a solid stateinfra-red radiation source; an element opaque to infra-red radiationpositioned between said source and said semiconductor device and havinga single aperture therein; means for moving the opaque element in timedrelation to the moving member; a transistorized amplifier having aplurality of transistor stages arranged to switch inversely in cascadebetween an on state and an off state so that a transistor of one stageis always conducting; means for connecting said amplifier to thesemiconductor device such that the stages of the amplifier switchinversely to produce a square wave voltage pulse in synchronism with thesquare wave voltage pulse generated by the semiconductor device;electrical storage means charged from a source responsive to theswitching off of the last stage of said amplifier; and means formeasuring the average value of the inrush charging currents to saidelectrical storage means so as to produce a direct reading of thefrequency of occurrence of the state of said moving member.
 2. Anapparatus according to claim 1, wherein the semiconductor devicecomprises a photo-transistor the base electrode of which is leftunconnected, and the solid stage infra-red source comprises a galliumarsenide lamp.
 3. An apparatus according to claim 2, further comprisinga zener diode connected in parallel with the gallium arsenide lamp andthe photo-transistor for stabilizing the voltage across said lamp andsaid photo-transistor, and a diode connected across theemitter-collector electrodes of the photo-transistor for ensuring cleanswitching thereof and for preventing reverse transients from causingmultiple switchiNg of the photo-transistor.
 4. An apparatus according toclaim 1, wherein said semiconductor device comprises a photo-transistorand transistor arranged as a darlington pair with commoned collectorelectrodes and the emitter electrode of the photo-transistor connectedto the base electrode of the transistor, and wherein a pair of seriesdiodes is connected between the commoned collector electrodes and thebase electrode of the first transistor of the amplifier.
 5. An apparatusaccording to claim 1, wherein the opaque element is a disc having a cutout portion which extends through 180*, whereby the square wave voltagegenerated by the semiconductor device has a mark-space ratio whichequals unity.
 6. An apparatus according to claim 1, wherein the laststage of said amplifier comprises a transistor which switches ''''on''''during the ''''off'''' period in which the infra-red radiation is cutoff from the semiconductor device and which switches ''''off'''' duringthe ''''on'''' period in which infra-red radiation reaches thesemiconductor device, the means for integrating the output of theamplifier includes an ammeter, a capacitor and a diode, the diode andammeter being connected in parallel with one another, and the capacitorbeing connected between the output of the last stage of the amplifierand the parallel combination of diode and ammeter so that theemitter-collector circuit of the transistor forming the last stage ofthe amplifier short circuits the capacitor and meter when said laststage transistor switches ''''on,'''' said capacitor being charged bythe current which passes through the ammeter when the transistor formingthe last stage of the amplifier switches ''''off.''''
 7. An apparatusaccording to claim 2, wherein said opaque element comprises a rotorhaving a cylindrical cup at one end with a single aperture therein, saidgallium arsenide lamp extending into said cup and being fixed to astator and said photo-transistor being housed in the wall of the statorin line with the rotating aperture in the cylindrical cup and saidgallium arsenide lamp.
 8. In combination, a road vehicle and anapparatus for determining the engine revolutions of the road vehicleincluding a semiconductor device sensitive to infrared radiation forgenerating a square wave voltage pulse by switching on when exposed tothe infra-red radiation and switching off when the infra-red radiationis cut off therefrom; a solid state infra-red radiation source; anelement opaque to infra-red radiation having a single aperture formedtherein, said opaque element being positioned between said source andsaid semiconductor device; means for moving the opaque element in timedrelation to the engine revolutions of said road vehicle; atransistorized amplifier having a plurality of inversely switchingtransistor stages, means for connecting said transistorized amplifier tothe semiconductor device such that the semiconductor device and thetransistor stages of the amplifier switch inversely in cascade betweenan ''''on'''' state and an ''''off'''' state so that a transistor of onestage is always conducting so as to produce a square wave voltage pulseevery time a pulse occurs from the output of the semiconductor device;and electrical storage means charged from a source responsive to theswitching off of the last stage of said amplifier; and means formeasuring the average value of inrush charging currents to saidelectrical storage means so as to produce a direct reading of the enginerevolutions of the vehicle.
 9. The combination according to claim 8,wherein the means for effecting a summation of the pulses from theoutput of the amplifier includes an ammeter, a capacitor, a diode, azener diode and a transistor, means for connecting said capacitor inseries with said amplifier across the collector-emitter path of thetransistor, means for connecting said diode across said ammeter andzener diode across the collector-emitter path of the transistor so thatthe transistor causes short circuiting of the capacitor when saidtransistor is turned ''''on'''' during the ''''off'''' period in whichthe infra-red radiation is cut off from the semiconductor device, saidcapacitor being charged by the current which passes through the meterwhen the transistor switches ''''off'''' during the ''''on''''period inwhich the infra-red radiation reaches the semiconductor device.
 10. Thecombination according to claim 8, wherein said opaque member comprises arotor having a cylindrical cup at one end with an aperture in the wallthereof, said solid state source being fixed to a stator and beinghoused within said cup, and the semiconductor device being housed in thewall of the stator in line with the solid state source and in the sameplane as the aperture in the wall of the cup.
 11. The combinationaccording to claim 8, wherein the semiconductor device comprises aphoto-darlington pair and the infra-red radiation source comprises agallium arsenide lamp, said combination further comprising a zener diodeconnected in parallel with a first series arm comprising the galliumarsenide lamp and a resistor, and a second series arm comprising aphoto-darlington pair and a resistor, the three armed parallel circuitso formed being connected in series with a resistor across a D.C.voltage source.
 12. An apparatus for determining the speed of a roadvehicle including a photo-transistor sensitive to electromagneticradiation which conducts when exposed to radiation and switches off whenthe radiation is cut off; a solid state radiation source; a disc, havinga cut away, portion positioned between the source and thephoto-transistor; means for rotating the disc in relation to enginespeed so that radiation from said source passing through the cut awayportion of said disc to said photo-transistor causes a correspondingseries of square wave voltage pulses to be produced at the output of thephoto-transistor; a transistorized amplifier having a plurality oftransistor stages, the stages of said amplifier being connected so as toswitch inversely in cascade in such a manner that at any one instant onetransistor of the amplifier is always conducting; means for connectingthe amplifier to the photo-transistor such that the output produced bythe amplifier is a square wave which is in synchronism with the squarewave voltage pulses produced at the output of the photo-transistor; andelectrical storage means charged from a source responsive to theswitching off of the last stage of said amplifier; and means formeasuring the average value of the inrush charging currents to saidelectrical storage means so as to provide a direct reading of the speedof the road vehicle.
 13. A device according to claim 12, wherein thesolid state radiation source comprises a gallium arsenide infra-redlamp.
 14. A device according to claim 12, wherein the cut away portionof the disc extends through 180* of the circumference of the disc.